Whitney first proposed embedding an elastomer with a conductive liquid for strain sensing. See, Whitney, R. J. “The measurement of changes in human limb-volume by means of a mercury-in-rubber strain gauge,” Proceedings of the Physiological Society 109 5P-6P (1949). Whitney filled a rubber tube with mercury to measure the change in circumferential girth of a human limb. Sixteen years later, Rastrelli, Anderson, and Michie filed a patent application, that issued as U.S. Pat. No. 3,304,528, for a more general design for an elastomeric strain gauge that included a broad range of materials. In 2007, Cheng, Chao, and Cheung filed a patent application, that issued as U.S. Pat. No. 7,500,399, for a strain gauge containing doped polymeric fluid. A recent embodiment of the “Whitney” strain gauge is polydimethylsiloxane (PDMS) rubber embedded with a microchannel of eutectic, gallium indium (eGaIn) conductive liquid. See, Dickey, M. D., Chiechi, R. C., Larsen, R. J., Weiss, E. A., Weitz, D. A., and Whitesides, G. M. “Eutectic Gallium-Indium (EGaIn): A Liquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature,” Advanced Functional Materials 2008 1097-1104. See, also, Kim, H. J., Son, C., and Ziaie, B. “A multiaxial stretchable interconnect using liquid alloy-filled elastomeric microchannels,” Applied Physics Letters 92 011904 (2008).
These strain gauges, however, can only sense extensional stretch, not transverse pressure or bending curvature. Additionally, existing pressure sensors and touch screens are composed of stiff, inorganic materials and polymers that limit flexibility and/or stretch, thus preventing biomechanical compatibility.
Emerging technologies, for example wearable computing, flexible tactile displays, and soft orthotics, can depend on stretchable sensors that register the location and intensity of pressure over a broad area. These “second skin” sensors are ideally conceived to maintain functionality even when stretched to several times their natural length. Additionally, they should also be soft enough to prevent significant interference with mechanics of human motion. Lastly, the sensors should be elastic and function repeatedly without hysteresis or permanent deformation.
Elastomer-based sensors, microelectronics, and artificial skin represent the next stage in a technological progression from rigid microelectronics to MEMS to soft microfluidics. While some emerging technologies have developed limited capabilities, many existing thin-film solutions are flexible but not stretchable. In addition, the next generation of sensor and circuits should preferably be able to conform to dramatic but reversible changes in shape and rigidity without interfering with the natural mechanics of a host.